JACKSON, Wyo. – A surge of monsoonal moisture has reached Western Wyoming with widespread rain and cool temperatures expected on Wednesday and Thursday. More seasonal weather will return over Labor Day weekend, though afternoon thunderstorms will remain possible.
Recent Weather
The previous week featured warm and dry weather, a significant uptick in wildfire activity, and an increase in cloud cover from Sunday to Tuesday at the leading edge of the monsoon moisture surge.
Wildfire smoke and poor air quality have also impacted the Jackson Hole Valley for the first time this year over the past few days, due to smoke from the Dollar Lake Fire near Green River Lakes.
On Tuesday, we saw a round of light showers in the morning, along with some additional light showers on Tuesday night. Rainfall was heavier on Tuesday and Tuesday night in Star Valley and Teton Valley compared to Jackson Hole.
Upcoming Weather Pattern
A widespread, soaking rain event is underway as of Wednesday morning. This will be the most significant rainfall for Northwest Wyoming and Eastern Idaho since the middle of May.
A surge of monsoonal moisture has impacted the Western U.S. over the past several days, bringing heavy rain and localized flash flooding from the Utah Canyons to the Sierra Nevada Range to Southern Idaho, including Boise, where flash flooding and mudslides were reported on Tuesday.
An upper-atmosphere disturbance (known as a shortwave) will slowly move across the Northern Rockies on Wednesday and Thursday, coinciding with the uptick in moisture to generate widespread rainfall across the Tetons and Northwest Wyoming.
Precipitable water, which is a measure of the amount of water vapor in the atmosphere, is about 3 standard deviations above normal for our area, or in the 99th percentile of what is considered normal. Dewpoints are also in the 50s, which is indicative of a lot of moisture for our area.
Wednesday looks like the wettest day with widespread rain expected, while Wednesday night through Thursday afternoon will feature more intermittent showers but with locally heavy downpours at times.
Lightning activity will be limited given the cool and relatively stable airmass in place, but some isolated lightning couldn’t be ruled out during the afternoons.
Rainfall amounts from Wednesday through Thursday will range from 0.5 to 1.5 inches across most of Teton County, with isolated higher amounts possible.
While mountains will generally see higher rainfall than the valleys, with summertime convective processes at work, there will be locally heavy pulses of rain that are less dependent on mountain orographics, which will lead to more random variation in totals.
NOAA has indicated a level 2 out of 4 slight risk for excessive rainfall and flash flooding in Teton County due to the unseasonably high levels of water vapor in the atmosphere.
However, the cool temperatures may ultimately temper rainfall rates enough to reduce the flash flood risk. But if we see any locally heavy showers or thunderstorms develop over an area, especially if training of heavy showers occurs (heavy rain showers or thunderstorms tracking over the same area), then some localized flooding couldn’t be ruled out in steep terrain or areas of poor drainage.
Recent wildfire burn scars will also have a higher risk of flash flooding and debris flow, as recently burnt soil is unable to absorb heavy rainfall in a short amount of time.
Overall, this is shaping up to be a much-needed soaking rain event for our area, given the ongoing drought conditions and fire concerns. But it will create less than ideal conditions for outdoor recreation with cool and wet conditions and wet and muddy trails.
On Friday, a transition day is expected with fewer and lighter showers and some breaks in the cloud cover are expected.
However, if we see enough sunshine and surface heating, that would lead to a more unstable atmosphere with a higher potential of afternoon thunderstorms which could produce dangerous cloud-to-ground lightning and locally heavy downpours.
The weather pattern is expected to warm up and dry out just in time for the Labor Day holiday weekend.
On Saturday, there will still be enough lingering moisture along with daytime heating for a chance of scattered afternoon thunderstorms. Conditions will be improved for outdoor recreation, but still be aware of the risk for afternoon lightning and rain.
Further drying is expected on Sunday and Monday with reduced thunderstorm potential. Initially, there were some hints of moisture and thunderstorm chances returning on Monday, but recent model runs have backed off on this.
Instead, moisture levels are projected to increase by Tuesday and Wednesday of next week with a chance of afternoon thunderstorms returning.
Here is a 7-day rainfall projection from the European Ensemble Model, which represents the average of 50 simulations of this model:
In terms of valley temperatures, highs will only reach the low 60s on Wednesday and upper 60s to near 70 on Thursday. A gradual but progressive warming trend is expected thereafter, with mid 70s on Friday, upper 70s on Saturday and Sunday, and low 80s on Monday.
Looking further out, confidence in the pattern is low beyond September 3. But most models have at least some monsoonal moisture creeping back into the area, which could result in thunderstorm chances on some days.
Temperatures are generally expected to be close to average (with some day-to-day fluctuations) during the first week of September, which means mid to upper 70s in the valley on most days.
Alan Smith, Meteorologist
